Digital signal integrity is of increasing importance as operating frequencies and bandwidth increase for digital devices, systems and networks (herein referred to collectively as “digital systems”). Some digital systems are capable of operating at digital frequencies that have significant spectral content in the microwave frequency range. The microwave frequency content requires greater precision at the design phase to accommodate microwave signal behavior in the digital system. In order to reduce design time and costs, it is advantageous to generate an accurate model of the digital system that can reliably simulate a response to a proposed high-speed digital signal. As one of ordinary skill in the art appreciates, a model generated from low frequency measurement data does not properly simulate high frequency performance of the modeled device.
Currently, digital designers use time domain tools and measurement systems, such as oscilloscopes and bit error rate testers to evaluate, qualify and develop models to simulate operation of digital systems. These traditional test tools provide a measurement that indicates whether a circuit operates properly or not under certain operating conditions, but does not provide information into a potential cause or source of any problem. Additionally, these tools cannot provide enough accurate test information that is useful in developing a model to accurately simulate unmeasured behavior. A bit error rate tester provides functional test information and can provide statistical information to provide an eye diagram and to determine jitter, but does not provide quantitative information for development of a model. A high speed digital scope provides quantitative information such as duty cycle distortion, inter-symbol interference and deterministic jitter, but the equipment is costly at the necessary frequencies, does not always provide sufficient bandwidth and dynamic range for accurate measurements, and does not provide enough information for development of a reliable model.
It is possible to make measurements of digital systems in the frequency domain using a vector network analyzer (“VNA”). Prior art frequency domain measurements make relative magnitude and phase measurements between a stimulus and response signal at microwave frequencies to characterize a device under test (herein “DUT”) and develop a high frequency model. The stimulus signal for the VNA measurement is a sine wave swept across a desired frequency range. Advantageously, frequency domain measurement technology provides sufficient bandwidth for accurate high frequency measurements and a calibration process that improves overall measurement quality by correcting for measurement artifacts that are not part of the digital system being measured. Measurements using a VNA are capable of accommodating imperfect signal integrity in the stimulus signal, isolating the response of the digital system to the imperfect stimulus and provide signal separation between incident and reflected signals in order to identify a source of signal degradation. VNA measurement technology also provides high dynamic range. In the case of digital system measurement, however, conventional VNA measurement technology has some limitations.
Digital devices operate in a saturated amplification state and therefore exhibit strong non-linearity. Traditional continuous wave measurements test in a linear region and are therefore useful, but incomplete for purposes of creating a simulation model of a digital system. Digital systems exhibit thermal and electrical memory effects meaning that a digital system response to complex signals is different from the response to the mathematical sum of the responses to individual spectral components when measured with a pure sinusoidal stimulus signal. At high speeds approaching the microwave range, the historical content of a digital signal stimulus can affect a response of the digital system at a single point in time. The presence of a frequency component of the stimulus signal can affect a digital system response for another frequency component present at the same time. In other words, some digital systems affect a response to one spectral frequency component in the presence of another spectral frequency component differently than if the other spectral frequency component were not present. An accurate pure sinusoidal stimulus does not simulate the DUT under operating conditions and, therefore, a measurement that uses just the pure sinusoidal stimulus signal cannot characterize an active digital DUT. Accordingly, it is not possible to accurately and reliably predict a digital system response to a complex signal from a series of measurements of the digital system response to individual sinusoidal signals that make up the complex signal in their linear operating regions. Therefore, the swept sine wave type of measurement that characterizes the prior art VNA measurement process is useful, but insufficient to properly stimulate an important effect of a digital system at microwave frequencies. It is advantageous in development of an accurate model of the digital system, to have data that reflects behavior in the non-linear operating regions and to reflect any memory effects of the digital system. There is a need, therefore, for a method and apparatus of measuring a frequency response of a digital system in the presence of a digital signal.